This invention relates to the field of depositing films or coatings on a substrate.
Thermal barrier coatings (TBCs) are a type of film that is deposited on a substrate. A variety of techniques have been examined to fabricate TBCs, but to date only two processing routes have been successfully scaled up to produce thick TBCs, typically about 250 xcexcm. These are the electron beam assisted physical vapour deposition (EB-PVD) and plasma spraying techniques which make extensive use of vacuum and sophisticated deposition equipment.
However, the above fabrication routes are expensive for large area units and mass production. Moreover, EB-PVD is a line-of-sight process which limits the capability of the method to coat three-dimensional components uniformly.
A new technique termed electrostatic spray assisted vapour deposition (ESAVD) is described in PCT/GB96/03105. ESAVD is a novel processing technique which offers a simple and cost effective method to manufacture ceramic films.
However, depositing the thick films that are generally required for TBCs has remained a problem.
This invention provides a method of depositing a material onto a substrate, comprising the steps of: feeding a material solution to an outlet to provide a stream of droplets of the material solution; applying a potential difference between the outlet and a substrate to electrostatically attract the droplets from the outlet towards the substrate; heating the substrate to provide an increase in temperature between the outlet and the substrate; and progressively increasing the temperature of the substrate during material deposition.
In the invention, it is recognised that when a thick film, especially an insulative film, such as that required for a TBC, is deposited, the insulating properties of the film tend to reduce the temperature of the material-receiving surface of the film with respect to that of the original substrate. So, while the substrate may be heated to a constant temperature, for example, by an electric back-heater on the rear face of the substrate so as to provide a temperature gradient from the substrate to the outlet, the actual surface of the deposited film, which receives the next layer of deposited material, can become progressively cooler.
To address this problem, the invention provides a progressively increasing substrate temperature so as to maintain a substantially constant temperature at the material-receiving surface.
This can lead to much more uniform properties through the deposition process, and so can lead to thick films, such as TBCs, being deposited with much more uniform, reliable and predictable thermal and mechanical properties.
As an example, the technique may be used for the fabrication of TBCs for gas turbine engines in power generation and aerospace applications. In addition, TBCs can also be used in automotive applications.
The present invention also provides an apparatus for depositing a material onto a substrate, comprising: an outlet; feeding means for feeding a material solution to the outlet to provide a stream of droplets of the material solution; a voltage source for applying a potential difference between the outlet and the substrate to electrostatically attract the droplets from the outlet towards the substrate; heating means for heating the substrate to provide an increase in temperature between the outlet and the substrate; a heating controller for controlling the heating means; and a temperature detector for detecting the temperature of a material-receiving surface on the substrate; wherein the heating controller is configured so as to be responsive to the temperature detector to control heating of the substrate to maintain a substantially constant temperature at the material-receiving surface.
Further respective aspects and features of the invention are defined in the appended claims.